Electrochemical devices include batteries, capacitors, electrochromic devices, etc. These are ionic devices employing ions as charge carriers. These ionic devices conventionally use a liquid such as water or an organic solvent as the medium that transports ions.
Take batteries as an example of the ionic devices, as devices such as cell phones become more compact and have higher performance these days, there has been an increasing demand for batteries as their power source. In particular, there has been a rapid advancement in the research and development for lithium ion batteries with a higher energy density and the commercialization thereof. However, since a lithium ion battery employs an organic solvent as the medium that transports ions, there is a relatively high possibility of leaking. In addition, since organic solvents are flammable, ignition is likely to occur in the case of leaking. In order to solve these problems regarding reliability of the battery, a study on all solid state lithium batteries has been carried out. For example, Japanese Laid-Open Patent Publication No. 2000-251939 discloses an all solid state battery employing a polymer solid electrolyte, and Japanese Laid-Open Patent Publication No. Sho 60-257073 and Japanese Laid-Open Patent Publication No. Hei 10-247516 disclose all solid state batteries employing an inorganic solid electrolyte.
In recent years, there has been an increasing study on thin film all solid state batteries. As the process of fabricating thin film all solid state batteries, vacuum thin film processes such as sputtering, ion plating and deposition are usually used (U.S. Pat. No. 5,338,625 and U.S. Pat. No. 5,141,614).
In the thin film all solid state batteries disclosed in the above prior art examples, particularly when lithium cobaltate is used as the positive electrode active material, a heat treatment in air or in an oxygen atmosphere is necessary after forming of thin films in order to increase the crystallinity of the active material. Accordingly, the substrate is usually made of highly heat-resistant quartz, alumina, silicon wafer, sapphire, etc. The substrates made of these materials, however, are thick and rigid. Since the energy density of battery is influenced by the volume, when a thin film battery with a small area is constructed, the percentage of the substrate in the battery is increased, rendering it difficult to ensure sufficient energy density.
In order to solve the problems described above and to develop the batteries with a high capacity and voltage, Japanese Laid-Open Patent Publication No. Sho 61-165965 proposes to form a plurality of solid electrolyte batteries on one substrate by patterning using a mask and to connect the plurality of solid electrolyte batteries in series or parallel.
Japanese Laid-Open Patent Publication No. Hei 8-64213 proposes to fabricate a thin battery comprising facing positive and negative electrode current collectors, which serve as jacket and accommodate a positive electrode active material, a solid electrolyte and a negative electrode active material interposed between the positive and negative electrode current collectors; to adhere the facing peripheries of the current collectors with a thermally adherent resin frame; and to integrate a plurality of the thin batteries by adhering the extended portions of the frame.
Furthermore, in order to use low-cost metal plates as the substrate since quartz, alumina, silicon wafer and sapphire are costly, U.S. Pat. No. 6,280,875 proposes to cover a metal plate with a metal oxide such as titanium oxide or zirconium oxide to protect the substrate from oxygen.
However, it is difficult to form a metal oxide layer, which is sufficient enough to protect the substrate from oxygen, at high temperatures of 600 to 1000° C. necessary to increase the crystallinity of the positive electrode active material.
Generally, metal oxides have an oxide ion conductivity. The higher the temperature increases, the more oxide ions a metal oxide conducts. For example, if a trace amount of Y2O3, CaO or Gd2O3 is added to ZrO2, the oxide ion conductivity will be about 10−3 to 10−2 S/cm. Accordingly, even if the substrate is covered with a metal oxide layer in order to protect the substrate, oxygen will reach the substrate during annealing of the positive electrode active material at a high temperature. If the substrate is made of an easily-oxidized material such as copper, the substrate will be oxidized and become brittle, rendering it difficult for the substrate to maintain its shape.